The present invention relates to a feed screw shaft support device which supports auxiliarily the intermediate portion of a feed screw shaft in order to avoid the resonance of the feed screw shaft produced due to the fact that the dangerous speed of the feed screw shaft goes into a low number of rotation area, when the support span of the feed screw shaft is long.
When the number of rotation of a feed screw shaft approaches the natural vibration frequency of the feed screw shaft, then resonance is produced, which is dangerous. This dangerous speed is in inverse proportion to the square of the support span of the feed screw shaft. When a table stroke is long such as in a numerically controlled surface grinder, then the feed screw shaft to feed a table is lengthened and thus the support span of the feed screw shaft is also lengthened, so that the dangerous speed goes into a low rotation area. As the feed speed is increased, the feed screw shaft is used at a high number of rotation, that is, the feed screw shaft must be used in the dangerous speed area thereof. In this case, there is known means in which an intermediate support to support the intermediate portion of the feed screw shaft is provided to shorten the support span of the feed screw shaft to thereby raise the natural vibration frequency of the feed screw shaft.
In one of conventional means of the above type, as shown typically in FIG. 5, between two bearings SA, SB respectively for supporting the two end portions of a feed screw shaft 2', there is interposed an intermediate support S which is free to move in a feed direction and has two feed screw shaft support portions A, B, a nut 3' movable by means of rotation of the feed screw shaft is interposed between the two feed screw shaft support portions A, B, and, when the intermediate support S is in engagement with the nut 3', one of the feed screw shaft supports of the intermediate support S is pushed by the nut 3' to cause the intermediate support S to move together with the nut 3', so as not to prevent the movement of the nut 3' (for example, Japanese Patent Unexamined Publication No. Sho. 58-109242).
Also, there has been conventionally known a feed screw shaft support device in which a feed screw shaft includes two end portions, the two end portions are respectively free to rotate, a nut is threadedly engaged with the feed screw shaft, and an intermediate support holding the nut therebetween is movable in the screw shaft direction, the intermediate support is formed as a slide bearing to thereby support the screw shaft, the intermediate support is supported by a linear guide device, and pulley blocks each having a running block are disposed in parallel between the intermediate support, base member, and nut to thereby link the intermediate support to the nut, so that the support span of the screw shaft can be reduced sequentially as the nut is moved (for example, Japanese Patent Unexamined Publication No. Hei. 2-66359.
However, in the technology disclosed in the above-mentioned conventional device, Japanese Patent Unexamined Publication No. Sho. 58-109242, as shown in FIG. 5, due to the fact that the intermediate support S interposed between the bearings SA and SB (distance 2L) can be moved after the nut 3' interposed between the support portions A and B (distance L) of the intermediate support S comes in contact with either of the support portions A and B, when the nut 3' is moved left from the condition shown in FIG. 5(a) in which the nut 3' has moved to one end portion of the intermediate support, that is, when the nut 3' is present between the support portions A and B, the support span A-SA is determined as a difference L between the bearings SA, SB distance 2L and the intermediate support portions A, B distance L and cannot be shortened any further, so that the dangerous speed cannot be reduced to a desired degree. Further, since one support portion of the intermediate support S must be contacted with the nut 3' before the intermediate support S can be moved, when the nut 3' comes in contact with the intermediate support S, the mass of the intermediate support S is suddenly and heavily added to the mass of the table to hinder the high speed movement of the table, causing the moving speed of the table to vary unfavorably.
Also, the technology disclosed in the above-mentioned conventional device, Japanese Patent Unexamined Publication No. Hei. 2-66359, has the following problems:
(1) Due to the fact that the intermediate support is guided only by a linear guide different from a guide for the table (lines 2 to 15, column 3, page 3, in the same publication), and that the dimension of the slider in the axial direction thereof is shorter than a dimension from the slider to the screw shaft support portion, the intermediate support is easy to fall down and the intermediate support has a poor mobility;
(2) Since the support of the screw shaft by the intermediate support is achieved by the slide bearing and thus there is produced a slight clearance in the screw shaft support portion, the vibration of the screw shaft cannot be controlled sufficiently;
(3) Because the intermediate support and table are guided by different guide members, it is difficult to set the respective guide rails to be parallel accurately. For this reason, an unreasonable force can be often applied to the screw shaft.
(4) Since the intermediate support and nut are connected to each other by means of a belt, the relative position relationship between the intermediate support and nut is apt to shift due to the slip of the belt, so that the given position relationship of the screw cannot be kept accurately.